1. Field of the Invention
The present invention pertains to artificial aging of aluminum alloy products, particularly to methods of artificially aging aluminum alloy products which include integration of the time and temperature effects on aluminum alloy products over an entire aging process.
2. Prior Art
Production of aluminum alloys includes casting of ingots which may be deformed into wrought products such as rolled plates, forgings or extrusions. The wrought product is solution heat treated by heating to one or more temperatures such as about 800 to 1100° F. to take substantial portions, preferably all or substantially all, of the soluble alloying elements (such as for an Aluminum Association (AA) alloy of the 7xxx series, zinc, magnesium and copper) into solution. After heating to the elevated temperature, the product is rapidly cooled or quenched to complete the solution heat treating procedure. Such cooling may be accomplished by immersion in a suitably sized tank of water or other liquid or by water sprays, although air chilling is usable as supplementary or substitute cooling means for some cooling. After quenching, certain products may be cold worked, such as by stretching or compression where feasible, to relieve internal stresses or straighten the product, even possibly in some products such as those of the AA 2xxx series, to further strengthen the wrought product. For instance, the product may be stretched 1 to 1½% or more, or otherwise cold worked a generally equivalent amount. A solution heat treated (and quenched) product, with or without cold working, is then considered to be in a precipitation-hardenable condition, or ready for artificial aging according to preferred artificial aging methods as herein described or other artificial aging techniques. As used herein, the term “solution heat treat”, unless indicated otherwise, shall be meant to include quenching.
After rapidly quenching, and cold working if desired, the wrought product is artificially aged by heating to an appropriate temperature to improve strength and other properties either alone or in conjunction with other processes such as mechanical or chemical treatment of the product. In one thermal aging treatment, the precipitation hardenable plate alloy product is subjected to two or more main aging steps, although clear lines of demarcation may not exist between each step. It is generally known that ramping up to and/or down from a given or target treatment temperature can itself produce aging effects which can be, and often needs to be, taken into account by integrating such ramping conditions and their precipitation hardening effects along with the main aging steps of the total aging treatment. Such thermal integration is described in greater detail in U.S. Pat. No. 3,645,804 to Ponchel, which is incorporated by reference herein. With ramping and its corresponding integration, two or three steps for thermally treating the plate product according to the aging practice may be effected in a single, programmable furnace and meet the targeted properties for the product.
Aging practices are known to impact the mechanical and physical properties of the product such as strength, fracture toughness and corrosion resistance. Generally, overaged products (products heat treated beyond a peak maximum strength) exhibit improved corrosion resistance and improved fracture toughness at the expense of loss of strength. The strength requirements for the product may be balanced against the need for corrosion resistance of the alloy, particularly for 7xxx series alloys used in aerospace applications which are subjected to corrosive environments.
The aging integration method described in the '804 patent is relevant only to the overaged conditions of the aging process and does not account for the impact of aging prior to the overaged state. The portion of the aging process having overaged conditions is represented by the aging data points of FIG. 1 (a plot of tensile yield strength versus time) that are to the right of the peak strength.
The prior thermal integration method of the '804 patent accumulates the time-temperature effects and signals that the aging process is complete for a desired property in the alloy when the accumulated thermal effect reaches a value known to be associated with the desired property in a particular alloy. The integration formula can be expressed asK=∫∫dEdtwhere K is a predetermined value for the alloy, E is a correction factor for each aging temperature and t is the period of time the alloy is at that temperature. The correction factor E can be expressed as   E  =            t      T              t              T        ′            where tT is the time required to achieve a desired property (e.g., strength) at a target temperature T and tT′ is the time required to achieve the same property at an arbitrary temperature T′. The E factor increases exponentially with temperature, yet the values of E are determined only for the overaged state of the alloy. No accounting is made for the thermal effects in the portion of the aging process where the alloy is in an underaged state, i.e. to the left side of the peak strength in FIG. 1.
According to the prior art method, aging at target temperatures is performed until the desired value of K is reached, with K having a predetermined correlation with strength. Strength per se is not calculated according to the prior art aging integration method, only the integrated value of K is calculated which is then correlated with strength. The starting point for that method is at the beginning of the overaging portion of an aging process, namely, at peak strength. The thermal effects of heating up an alloy and aging steps imposed before reaching peak strength are not considered. The K value is a measure of change in the thermal effect on the alloy (the time spent at each temperature) after peak strength is achieved and ranges from near zero (at peak strength) to a positive number (at reduced strength from overaging). The K value does not represent an actual property in the alloy.
In an effort to compare the thermal effect (K value) of the prior art method with actual strength, a value of strength for an overaged alloy correlated from calculations of K according to the prior art aging integration method was plotted over time in FIG. 1. The overaged portion of the curve exhibits some similarity to the actual strength of the alloy. According to such a correlation, in the underaged portion of the curve, the K value would be nearly zero and predicted strength would approach a maximum. See the prior art plot in FIG. 1. However, experience shows, as indicated by the data points of measured strength to the left of peak strength in FIG. 1, that yield strength begins low and increases during the underaged state of the alloy to a peak value and then decreases in the overaged portion of the aging process. The difference between the actual tensile yield strength (plotted data) and the tensile yield strength that would be determined based on the correlations used in the prior art model in the underaged portion of the graph represents an inaccuracy in the prior thermal integration method. Not only does the prior art method fail to predict an alloy property (e.g. strength), it does not account for the thermal effects of the entire aging process which includes the underaged portion.
Accordingly, a need remains for a method of integrating all of the thermal effects of artificial aging on properties of aluminum alloys that accounts for the entire artificial aging process (including the underaged portion) and allows for the calculation of properties of aged alloys.